1. Field of the Invention
This invention relates to a beam shifting device for shifting a beam so as to direct it exactly toward a fixed object.
2. Description of Related Art
Lasers, which are used as light sources for various kinds of optical apparatuses, such as exposure apparatuses and holographic apparatuses, generate beams having a strong directivity. However, even such a laser beam suffers what is called a "beam shift," which are attributed to internal causes, such as thermal changes, and external causes, such as thermal stress, in association with a resonator. The term "beam shift" as used herein shall mean and refer to a parallel deviation of a laser beam from an intended beam path.
Typically, in order to cancel a beam shift from an intended beam path, a double mirror beam shifting device is well known in the art, as shown by an example in FIG. 8. This device has two reflection mirrors 100 and 101 with normal lines of their reflection surfaces intersecting at right angle, which are oscillated so as to direct a light beam Lb travelling from a light source 102, such as a laser, toward a fixed object 104. Such a device is usually provided with ne or more slits 103a and 103b in the intended optical path Ox. If the laser beam Lb shifts or deviates from the intended optical path Ox, the mirrors 101 and 102 must be cooperatively regulated in angle relative to the light beam Lb. Such cooperative angular regulation of the mirrors 101 and 102 is very difficult. In addition, since the shift or deviation of light beam Lb is not parallel with the intended optical path Ox, regulation of the mirror 101 for making a light beam Lb pass through the upstream slit 103a neither allows the light beam Lb to pass the downstream slit 103b nor directs it perpendicularly to the object 104.